Electrical terminals or other devices having contact surfaces are frequently electroplated with a metal which has superior contact properties as compared to the basis metal, for example, it is common to plate copper terminals with nickel and with a thin gold plating over the nickel to function as the contact surfaces. Tin is also frequently used as a plating for terminals or other contact devices.
The ideal, and nearly perfect, contact surface is electroplated gold but other metals, particularly tin, are frequently used because of the very high cost of gold. Electroplated nickel has been used on terminals and other contact devices to a limited extent as a contact surface (rather than as an underplating for gold) but its use has been very limited for the reason that when a nickel surface is exposed to the atmosphere, it develops a strong insulating oxide film which must be broken when the contact device is placed in service as happens when the contact device is crimped onto a wire or brought into engagement with a complementary contact device. A tin plating will also develop an oxide film but the tin oxide is easily broken by a relatively low contact force when the device is placed in service for the reason that the underlying metallic tin is relatively soft and the hard tin oxide on the surface is fractured under a relatively low contact force. Nickel, however, is relatively hard and it is relatively difficult to break the insulating film when a nickel plated terminal is placed in service.
Nickel has been used in the past on contact surfaces where the normal force (the contact force exerted on the contact surface) is 500 grams or greater but has not been considered suitable for contact surfaces when the normal force is much below 500 grams. Many electrical terminals are designed to produce a normal force of about 100 grams and a higher normal force is not practical because of the small size of the terminals and the limitations of the metal from which the terminals are manufactured. For example, many of the commonly used types of connecting devices for integrated circuit chip carriers contain contact terminals which, because of size limitations, exert a normal force in the 100 to 200 gram range and it is generally regarded as necessary to use a gold contact surface on such terminals notwithstanding the added cost.
The foregoing comments apply to nickel platings produced by conventional plating techniques which will ordinarily have a relatively random orientation of the crystallites in the plating.
We have found that if a electroplated nickel surface on a terminal device has a particular grain orientation, the development of the oxide film on the nickel surface will take place only to a limited extent, and as a result, a stable low resistance electrical connection will be obtained when the plated device is placed in service. Specifically, if the electroplated surface has about 50 percent of the volume fraction of the crystallites therein preferentially oriented so that their {100} atomic planes are substantially parallel to the contact surface itself, the characteristics of the connection will be outstanding and for many purposes will be comparable to a gold plated contact.